In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a catalyst comprising a silver component, usually with one or more further elements deposited therewith on a support. The olefin oxide may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
The performance of the epoxidation process may be assessed on the basis of the selectivity, the catalyst's activity and stability of operation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. The catalyst is subject to an ageing-related performance decline during normal operation. The ageing manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity, however at the expense of selectivity. In the typical operation of a fresh catalyst, the process is operated at a reaction temperature of at most 255° C. Upon catalyst ageing the reaction temperature may gradually be increased to values substantially above 255° C. until the reaction temperature becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged. It goes without saying that from an economical point of view it is highly desirable to improve the performance of the catalyst and to extend its lifetime as much as possible. Quite modest improvements in the maintenance of selectivity over long periods yields huge dividends in terms of efficiency in the olefin epoxidation process and, if applicable, also in the overall process for the production of a 1,2-diol, a 1,2-diol ether or an alkanolamine.
Therefore, for decades much research has been devoted to improving the activity, the selectivity and the lifetime of the catalysts, and to find process conditions which enable to fully exploit the catalyst performance.
An organic halide, for example a chlorohydrocarbon, may be added to the feed to an epoxidation reactor as a reaction modifier for increasing the selectivity. The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide.
U.S. Pat. No. 4,766,105 and U.S. Pat. No. 4,761,394 disclose that rhenium may be employed as a further element in the silver-containing catalyst with the effect that the selectivity of the olefin epoxidation is increased.
U.S. Pat. No. 6,372,925 and WO-01/96324 teach that when operating with a catalyst which exhibits an improved selectivity, for example a catalyst known from U.S. Pat. No. 4,766,105 or U.S. Pat. No. 4,761,394, it is advantageous to increase the ethylene content of the feed when the catalyst has reached a certain stage of ageing. The increased ethylene content substantially improves the selectivity of the conversion of ethylene into ethylene oxide.
Not withstanding the improvements already achieved, there is a desire to further improve the performance of the silver-containing catalysts in the production of the production of an olefin oxide, a 1,2-diol, a 1,2-diol ether or an alkanolamine.